At start of print only one z axis is working

Hi,
I got back to 3d printing after nearly a year a few days ago. My first prints went fine but today I have a problem and no clue how to solve it

I still use printrun and when i start printing, my nophead mendel90 moves to the home position on all three axes just fine. Then it starts moving down the z axis and shortly after it starts moving down only one z axis motor works and the other one seems to be "blocked". I can then manually rotate the second z axis and it starts spinning... but then the first motor instantly stops. If i rotate the first one again the second one stops again ... and so on and on. It doesn't matter if I want to print through USB or from the SD card.

When using printrun manually I can move the z axis up and down just fine.

Quotehttp://reprap.org/wiki/Melzi
Melted screw terminals
Sometimes, the screw terminals on a Melzi melt. More often than not, it's the bed terminal, and is due to the wire being poorly connected, ie loose in the screw terminal. It can also cause the MOSFET to fail, as it cannot dissipate heat generated as it switches on and off. RepRapPro advises customers to check the screw terminal connections periodically: [reprappro.com]
If the Melzi PCB is undamaged, you can simply replace the screw terminal. There are a couple of different screw terminal blocks, these are the most common:
This is the most common one - [uk.farnell.com]
this one is a bit taller, and easier to get the wires into - [uk.farnell.com]
The heated bed power cable carries about 10A at 12V, so any poor connection will tend to heat up. Check that your power and heated bed wires are stripped back far enough to go well into the screw terminals, and it's always worth checking that the screw terminals are well-tightened occasionally. Also, wire the printer with enough slack on the heated bed power wire so that it can be attached to frame, and there shouldn't be any movement of it at the Melzi end. But strain relief on any moving wire is necessary - try to make the movement in the actual wire, rather than at a connection point, to avoid the wire fatiguing.